The avian-human (AH) influenza A/Mallard/NY/6750/78 and ca A/Ann Arbor/6/60 reassortant viruses were compared for safety, attenuation, infectivity, immunogenicity, and efficacy against attenuated virus challenge in test performed in fully susceptible infants and children, 6-48 months of age. The reassortant viruses were derived from the wild type human influenza A/Bethesda/1/85 (H3N2) or influenza A/Kawasaki/9/86 (H1N1) virus. The ca and AH reassortants derived from the same wild virus type donor were similar with respect to attenuation, immunogenicity, infectivity and efficacy. However, both AH and ca H1N1 reassortants appeared to be more infectious than their H3N2 counterparts. A single infection induced complete resistance to subsequent challenge with the homologous attenuated virus. However, 55-60% of vaccinees developed a significant antibody response to a second dose of vaccine. Reassortants prepared from two avian influenza A virus donors, A/Mallard/NY/6750/78 and A/Mallard/Alberta/88/76, and the same human influenza A/Bethesda/85 (H3N2) wild type virus were compared in seronegative adult volunteers. The reassortants appeared to be similar, hence, future efforts will be directed toward the further evaluation of reassortants derived from the better characterized A/Mallard/NY/6750/78 donor virus. A dose response study was performed to evaluate the attenuation, immunogenicity, infectivity and efficacy of ca reassortant influenza B/Ann Arbor/1/86 virus in seronegative young adults. The level of attenuation, immunogenicity, and efficacy were satisfactory. In immunogenicity tests performed in elderly persons (65-83) simultaneous administration of live influenza a vaccine and inactivated influenza virus vaccine was superior to either vaccine along. The ELISA test for detecting immune responses to virus infection was modified to utilize new spectrophotometers which can measure the rate of color development in microtiter wells. This new kinetic based ELISa requires only a single dilution of specimen and yields results which are comparable in terms of sensitivity and specificity to those produced by older endpoint ELISA's which require multiple dilutions.